JPH05226634A - Integrated type photodetector and coherent light communication system equipped therewith - Google Patents

Abstract

PURPOSE:To realize an integrated photodetector with a configuration with an improved S/N and a quick response and a coherent light communication system using the detector. CONSTITUTION:In an integrated photodetector, a photodetector of FET structure (MESFET structure, JFET structure, etc.) is integrated monolithically, thus constituting a balanced receiver. Furthermore, by integrating a 3dB coupler 6 monolithically, a signal light and a local emission are coupled and are equally distributed here and then are coupled to two photodetectors with nearly an equal coupling efficiency through a ridge waveguide 5, thus suppressing noise and obtaining a value which is close to an ideal S/N. Since these elements are integrated on a same substrate, same coupling can be constantly made stably.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an integrated photodetector including a balanced receiver used for coherent communication and the like, and a coherent optical communication system including the same.

[0002]

2. Description of the Related Art FIG. 12 shows the structure of a conventional integrated photodetector. In this structure, a non-doped (φ−) GaAs buffer layer 202 and a non-doped Al _x Ga _{1-x As} 203 are laminated on a GaAs semi-insulating substrate 201 to form a GaAs
_{60Å / Al y Ga 1-y} 0.9μm superlattice of As30Å
The degree or _{φ-Al y Ga 1-y} As204 several μm laminated,
A non-doped Al _x Ga _1-x A layer is formed on top of it as a cladding layer.
0.3 μm of s205 is laminated. After forming the ridge waveguide, the n-GaAs active layer 206 is laminated with a thickness of 0.2 μm and an impurity concentration of 1 × 10 ¹⁷ cm ⁻¹ and patterned, and the source electrode 211, the drain electrode 213, and Put on the gate electrode 212.

However, such a conventional example has the following drawbacks when performing optical heterodyne detection.

In the case of optical heterodyne detection, the electric field of the signal light and the local light is E _s = √P _s · exp [j (ω _s t + φ _s )] E _LO = √P _LO · exp [j (ω _LO t + φ _LO ). ] P _s , P _LO : Power of signal light, local light ω _s , ω _LO : Angular frequency of 〃 φ _s , φ _LO : Phase of 〃 ε: Coupling efficiency for signal light of the multiplexer After that, the photocurrent i obtained by the photodiode or photodetector is i = (2πeη / hω) {εP _s + (1-ε) P _LO +2 [ε (1-ε) P _s P _LO ] ^{1 / 2} · cos [(ω _LO −ω _s ) t + (φ _LO −φ _s )]} + i
_When the local oscillation light has no intensity noise and the local oscillation current is sufficiently larger than the signal light, the noise current i _n (t) becomes <i ² _n > = 2e ² · 2πη / hω · (1-ε) P _LO . Therefore, when the signal-to-noise ratio S / N is expressed using the number of counts per pulse width, (S / N) = [4ε (1-ε) N _LO N _s ] / [2 (1-ε) N _LO + 2z] → 2N _s (ε → 1, (1-ε) N _LO >> z). Here, N _LO and N _s are signal lights per pulse,
The count number of local light, z indicates thermal noise (resistance noise).

In coherent optical communication using a semiconductor laser, the local oscillation power is not sufficiently high, and local oscillation gain that suppresses thermal noise cannot be obtained (that is, (1-
ε) N _LO >> z does not hold). Further, noise larger than shot noise (shot noise) appears in local light (amplitude noise is large).

In order to solve these two problems which cannot be solved by the conventional example shown in FIG. 12 and the like, it is common to use two photodetectors to form a balanced receiver for heterodyne detection. Is becoming

FIG. 9 shows the structure of a balanced receiver. If the transmittance of the beam splitter is 50%, the two photodiodes have a signal current of the same amplitude and an opposite phase, respectively.
A noise current flows due to fluctuations in local oscillation amplitude having the same amplitude and the same phase. When this is added by a differential circuit, the signal currents are added, the local oscillation amplitude noises cancel each other out, and the shot noises add up. Therefore, an ideal SN ratio can be realized. In this case, it is important that the characteristics of the two photodetectors constituting the balanced optical receiver are uniform.

With such a configuration, better results are obtained in terms of variations in quantum efficiency and electrostatic capacitance, as compared with the combination of FETs alone shown in FIG. Further, since the relative positional relationship between the two photodetectors is also determined, it is possible to improve the balance of the optical coupling by connecting the two fibers together in the optical fiber connection.

Further, as a balanced receiver used in coherent optical communication, an integrated receiver of two pin photodiodes has been reported (O. WADA e.
tal Electronics Letters V
ol. 24, No. 9, p. 514 (1988)). FIG. 13 shows this conventional example.

Due to the embedded planarization technique, the semi-insulating property I
Two front-illuminated pin photodiodes are integrated on an nP substrate 301. Photodiodes having a light receiving diameter of 20 μm are arranged at intervals of 500 μm, and isolation grooves 302 are formed between the light receiving elements by etching for element isolation.

In the conventional example shown in FIG. 13 as well, better results are obtained in terms of variations in quantum efficiency and electrostatic capacitance than in the combination of pin photodiodes alone. Also, pin
Since the relative positional relationship of the photodiodes is also fixed, it is possible to improve the balance of optical coupling by connecting two fibers together in the optical fiber connection.

[0012]

[Problems to be Solved by the Invention] However, 2
In the above-described conventional example of the configuration of the balanced photoreceiver composed of two photodiodes, since coupling is performed by each photodetector and the fiber, the coupling efficiency to the two photodetectors is different, and the element Even if a product having uniform characteristics is manufactured, the balance is lost at the coupling portion, and thus there is a problem that noise cannot be suppressed sufficiently.

Further, the conventional example shown in FIG. 13 has the following problems. (1) Since it is difficult to manufacture an element in which the characteristics of the two photodetectors are uniform, it is not possible to sufficiently suppress noise. (2) In the heterodyne detection, since the intermediate frequency component having a higher frequency than the signal light is detected, the detector needs to have a wide band. Therefore, in the conventional example of FIG. 13, the detector is used for communication with a high pit rate. I can't. (3) Since it is a vertical device (the direction of electric field and current is orthogonal to the film thickness direction), it is difficult to integrate it with the FET that is a preamplifier. Therefore, an object of the present invention is to provide an integrated photodetector and a coherent optical communication system using the photodetector, which solve the above problems.

[0014]

According to the first aspect of the present invention, the signal light and the local light are integrated by monolithically integrating the 3 dB coupler in the balanced receiver in which the FET structure photodetector is monolithically integrated. After the emitted lights are combined and equally distributed, they are combined into two photodetectors with almost equal coupling efficiency, and noise can be suppressed to obtain a value close to an ideal signal-to-noise ratio. In addition, since these elements are integrated on the same substrate, stable and same coupling can always be performed, and integration with other optical devices (for example, local light source) is facilitated. There is.

According to the second configuration of the present invention, a monolithically integrated balanced type receiver is manufactured by using a photodetector with an FET structure (MESFET structure, JFET structure, etc.), and the following characteristics are obtained. can get. (1) Since two photodetectors having uniform characteristics can be easily manufactured, noise can be suppressed and a value close to an ideal SN ratio can be obtained. (2) Since the FET structure photodetector can respond at high speed, it can be used for communication with a high bit rate. (3) Since the FET and the preamplifier can have the same layer configuration, the integration with the preamplifier is easy, no extra resistance and capacitance are generated, and the band is not deteriorated.

Further, according to the optical communication system of the first configuration of the present invention, in the one-way coherent optical communication system in which the transmitting end station and the receiving end station are connected by the optical transmission line, The heterodyne detection is performed by using the integrated photodetector having the above configuration.

Further, according to the optical communication system of the second structure of the present invention, in the bidirectional coherent optical communication system in which the transmitting end station and the receiving end station are connected by the optical transmission line, The heterodyne detection is performed by using the integrated photodetector having the above configuration.

[0018]

[Embodiment 1] FIG. 1 shows a first embodiment of a structure in which a balanced receiver using an FET structure photodetector and a 3 dB coupler are integrated.

On the semi-insulating substrate 1 of GaAs, a .phi.-GaAs layer 2 as a buffer layer having a thickness of 0.5 .mu.m is laminated, and
The 1 GaAs layer 3 is laminated in a thickness of 1.5 μm and used as a waveguide layer φ-
AlGaAs (thickness 30Å) and φ-GaAs (thickness 60)
The multi-quantum well (MQW) 4 of Å) is laminated by 0.39 μm, and the φ-AlGaAs layer is formed by 0.3 as the second cladding layer.
μm is laminated.

Next, the ridge waveguide 5 and the 3 dB coupler 6 are manufactured by the following process. (1) Pattern the waveguide and the 3 dB coupler by photolithography. (2) 0.28 μ of φ-AlGaAs in the second cladding layer
m by the reactive ion beam etching (RIBE) using Cl ₂ gas, the waveguide 5 and 3 dB
The coupler 6 is formed. (3) The resist mask is peeled off and the surface is carefully cleaned.

On the thus-fabricated three-dimensional waveguide, as the active layer 7, Si of 1.0 × 10 ¹⁷ cm ⁻³ was added.
Re-grow the GaAs layer 0.2 μm. As a crystal growth method for this structure, a molecular beam epitaxy method (MB
E), organometallic CVD method (MO-CVD), and the like.

Next, the light receiving portion is manufactured. FET is FET
Because of the structure, the source electrode 11 and the drain electrode 13 are Au-G as ohmic electrodes for detecting the absorption carriers.
Using e / Ni / Au, Al is deposited as the gate electrode 12. The element size is a gate length of 4 μm, a gate width of 100 μm, a source-gate interval of 1 μm, and a gate-drain interval of 4 μm.

Next, the device process of the light receiving section will be described. (1) n-GaA of the active layer 7 by photolithography
s is selectively etched. As an etching method,
In the case of wet etching, hydrogen peroxide (200 cc) and aqueous ammonia (1 cc) are used as etchants, and in the case of dry etching, reactive ion etching (RIE) using CC1 ₂ F ₂ is used.
As a result, only GaAs is selectively etched. (2) The sample is heated and silicon oxide is vapor-deposited by a spatter vapor deposition device. (3) A portion other than the active layer 7 is covered with a resist by photolithography and etched with buffer hydrofluoric acid (BHF). (4) φ-Al on the surface other than the surface of the active layer 7 and the waveguide 5.
CVD of silicon nitride to protect the surface of GaAs
Vapor deposition by the method. (5) Source / drain electrode patterning is performed. (6) The silicon nitride of the source / drain electrodes is etched by RIE using CF ₄ gas. (7) Au-Ge / Ni / Au is continuously vapor-deposited. (8) Au-Ge / Ni / Au other than the electrode part is formed by heating the Az remover and peeling the resist.
To remove. (9) Alloying is performed to make ohmic contact between the active layer 7 and the electrode. (10) Gate electrode patterning is performed. (11) The silicon nitride in the gate electrode portion is etched by RIE using CF ₄ gas. (12) Electron beam (EB) vapor deposition of Al as a gate material. (13) By heating the Az remover and peeling the resist, Al other than the gate electrode portion is removed. (14) The semi-insulating substrate 1 is mechanically lapped so that the thickness of the entire device is 100 μm to 150 μm. (15) Au-Ge / Ni / Au is continuously vapor-deposited. (16) Alloying is performed to improve the adhesion with the substrate 1. With the above process, the balanced receiver and the 3 dB coupler 6 can be integrated.

Next, the operation of this element will be described. FIG. 2 shows the configuration of heterodyne detection using a balanced receiver.
Signal light is incident on one side of the 3 dB coupler 6 and local light is incident on the other side. In the coupling region, the signal light and the local light are coupled, and 2
Almost equally distributed to the waveguides 5 of the book. The equally distributed light propagates through the waveguide 5 as it is, reaches two photodetectors, and is detected. A positive electric field V _D is applied to the drain electrode 13 of the photodetection FET with respect to the source electrode 11, and the gate electrode 1
2, a negative electric field V _G is applied to the source electrode 11.
Since the gate electrode 12 is a Schottky electrode, the depletion layer extends into the active layer 7. This depletion layer width changes depending on the gate voltage V _G , and the channel width changes accordingly. As a result, the current _ID flowing between the drain electrode 13 and the source electrode 11 changes. When the depletion layer reaches the layer below the active layer 7, the channel is closed and the current I _D stops flowing. When light comes in this state, the guided light is guided from the waveguide 5 below the active layer 7 while being absorbed by the active layer 7, and carriers are generated there. Among the generated carriers, the electrons reach the drain electrode 13 by the electric field and are detected. Also,
The holes are attracted to the depletion layer, and as a result, the depletion layer contracts.

As described above, when light is not irradiated, the channel is pinched off, so that no current flows, but the current flows because the depletion layer contracts due to light. The depletion layer changes according to the irradiation light amount, a current flows between the source and drain, and the light amount is detected.

From the two photodetectors, a signal current having the same amplitude and the opposite phase and a noise current caused by the fluctuation of the local amplitude having the same amplitude and the same phase flow. By adding with a differential circuit, the local amplitude noise can be removed and a value close to an ideal SN ratio can be realized.

In this embodiment, the FET structure photodetector is 3d.
Since it is monolithically integrated with the B coupler 6, the light coupled to the two photodetectors can be made almost equal,
Noise is suppressed, and a value close to the ideal SN ratio is obtained.
In addition, since they are integrated to form stable bonds and branches, stable characteristics can be obtained. In addition, FET
Since it has a structure, it can be easily integrated with the preamplifier, and a better SN ratio can be obtained.

The material in this embodiment is GaAs / Al.
The effect is not limited to GaAs, and similar effects can be obtained when other III-V group compound semiconductors are used or when a IV group semiconductor such as Si is used for the substrate.

[0029]

[Embodiment 2] FIG. 3 shows a balanced receiver using an FET structure photodetector 21, a 3 dB coupler 22 and a local light source 23.
The integrated structure is shown. The local light source 23 may be any laser provided with an output waveguide, but here, for example, a case where a laser having a distributed reflection (DBR) structure is integrated will be described.

FIG. 4 shows the layer structure. The local light source 23 is n
-On the GaAs substrate 25, n-Al _0.3 Ga _{0.7 As} 26
2 μm, p-GaAs active layer 27 0.2 μm, p-
Al _0.2 Ga _0.8 As 0.1 μm, p-Al _0.07 Ga
_0.93 As29 0.1μm laminated, grating 30
To make. After that, the region other than the laser is etched up to the substrate 25.

Next, in order to manufacture the photodetector 21, p-
Al _0.3 Ga _{0.7 As} 31 2 μm, φ-Al _0.1 Ga _0.9
As32 is 2 μm, φ-Al _0.3 Ga _{0.7 As} 33 is 0.
2 μm and n-GaAs 34 are laminated to 0.2 μm. The layer structure of the photodetector 21 is as shown in FIG.

Next, the operation of this element will be described. FIG. 2 also shows the configuration of the second embodiment of heterodyne detection using a balanced receiver. Signal light propagates to one side of the entrance of the 3 dB coupler 22, and local light propagates from the local light source 23 monolithically integrated on the same substrate 25 to the other side. Since the local light source 23 is monolithically integrated, local light can be obtained with high efficiency and stability. The signal light and the local light are coupled in the coupling region and are substantially equally distributed to the two waveguides. The equally distributed light propagates through the waveguide as it is and 2
The number of photodetectors 21 reaches and is detected. Photo detector 21
The operation of is the same as that of the first embodiment.

In this embodiment, since the balanced receiver composed of the FET structure photodetector is monolithically integrated with the 3 dB coupler and the local light source, the light coupled to the two photodetectors is almost equal. In addition, since local light can be obtained with high efficiency, noise is suppressed and a value close to an ideal SN ratio can be obtained. Further, by being integrated, the coupling or branching between the local light source and the coupler and the coupler and the photodetector becomes stable, and the characteristics become stable. Also, F
Since it has the ET structure, it can be easily integrated with the preamplifier, so that a better SN ratio can be obtained.

Also in this embodiment, the material is GaAs / AlGa.
The effect is not limited to As, but the same effect can be obtained when another III-V group compound semiconductor is used or when a IV group semiconductor such as Si is used for the substrate.

Further, in this embodiment, the local light source 23 is provided with a wavelength tuning function, so that it is possible to select and detect a desired wavelength from multiple wavelengths.

[0036]

[Embodiment 3] FIG. 5 shows the structure of a third embodiment of a balanced type receiver manufactured by integrating photodetectors of the waveguide type FET structure.

On the GaAs semi-insulating substrate 41, a .phi.-GaAs layer 42 as a buffer layer is laminated in a thickness of 0.5 .mu.m, and a .phi.-AlGaAs layer 43 as a first cladding layer is 1.5 .mu.m.
m-layered, φ-AlGaAs 30Å as the waveguide layer 44
And a MQW layer of φ-GaAs 60 Å are laminated by 0.39 μm, and a φ-AlGaAs layer 45 is laminated by 0.3 μm as a second cladding layer.

The manufacturing process of the ridge waveguide and the device process of the light receiving portion are substantially the same as those in the first embodiment. In FIG. 5, 46 is an n-GaAs active layer, 47 is a silicon nitride film, 51 is a source electrode, 52 is a gate electrode, and 53 is a drain electrode.

The above process makes it possible to easily make a balanced receiver. The operation of each element is the same as that of the first embodiment. FIG. 2 already mentioned shows the configuration of the heterodyne detection using the balanced receiver. When the signal light and the local light are mixed and branched by the coupler and are incident on the two FET structure photodetectors, a noise current due to the fluctuation of the local amplitude and the signal current of the same amplitude and the opposite phase flows. By adding up with a differential circuit, local oscillation amplitude noise is removed, and ideal SN
Values close to the ratio can be achieved.

In this embodiment, since the FET structure photodetector can respond at high speed, it can be used in high bit rate communication. In addition, since the fabrication is easy and the uniformity is good, S
The N ratio becomes a value closer to the ideal SN ratio. Further, since it is a waveguide type, it can be easily integrated with other optical devices (optical multiplexer / demultiplexer, laser, etc.), and a better SN ratio can be obtained. Further, since it has the FET structure, it can be easily integrated with the preamplifier and a better SN ratio can be obtained. The material is the same as in the above example.

[0041]

[Embodiment 4] FIG. 6 shows the structure of a balance receiver according to a fourth embodiment, which is manufactured by integrating surface-illumination type FET structure photodetectors.

On the semi-insulating substrate 61 of GaAs, a φ-GaAs layer 62 as a buffer layer is laminated in a thickness of 0.5 μm.
-AlGaAs layer 63 is laminated by 1.5 μm, and active layer 64 is formed.
As Si, a 1.0 × 10 ¹⁷ cm ⁻³ n-GaAs layer is regrown to 0.2 μm. Molecular beam epitaxy (MBE) and organometallic C are used as crystal growth methods for this structure.
There is a VD method (MO-CVD) or the like. In FIG. 6, 65
Is a silicon nitride film, 71 is a source electrode, 72 is a gate electrode, and 73 is a drain electrode.

The manufacturing process of the light receiving portion is substantially the same as that of the first embodiment. This process makes it possible to easily make an integrated balanced receiver. The operation of each element is the same as that of the first embodiment except that light is emitted from the upper surface of the detector.

The structure of the heterodyne detection is shown in FIG. 2 similarly to the third embodiment. When the signal light and the local light are mixed and branched by the coupler and are incident on the two FET structure photodetectors, a noise current due to the fluctuation of the local amplitude and the signal current of the same amplitude and the opposite phase flows. By adding them together with the differential circuit, the local oscillation amplitude noise can be removed and a value close to an ideal SN ratio can be realized.
Also in this embodiment, since the FET structure photodetector can respond at high speed, it can be used in communication with a high bit rate. Moreover, since the fabrication is easy and the uniformity is good, the SN ratio becomes a value closer to the ideal SN ratio. Moreover, since the two photodetectors are integrated, the coupling θ efficiency with the fiber or the like is improved. Further, since it has the FET structure, it can be easily integrated with the preamplifier, so that a better SN ratio can be obtained. The same can be said for the materials as in the above embodiment.

[0045]

[Embodiment 5] FIG. 7 shows a fifth embodiment of the structure in which the FET structure waveguide type photodetector and the preamplifier FET are monolithically integrated.

On the GaAs semi-insulating substrate 81, a .phi.-GaAs layer 82 is laminated as a buffer layer in a thickness of 0.5 .mu.m, and a .phi.-AlGaAs layer 83 is formed as a first cladding layer in a thickness of 1.5 .mu.m.
m-layered, and φ-AlGaAs30Å as the waveguide layer 84
And an MQW layer of φ-GaAs 60 Å are laminated by 0.39 μm, and a φ-AlGaAs layer 85 is laminated by 0.3 μm as a second cladding layer.

The manufacturing process of the ridge waveguide and the device process of the light receiving portion are substantially the same as those in the first embodiment. By this process, the FET for light detection can be manufactured, but at the same time, the FET for preamplifier can also be manufactured by not providing the ridge waveguide below the active layer 86. In this embodiment, two photodetectors shown in FIG. 9 and a preamplifier are integrated.

The operation of the photodetector is similar to that of the above embodiment. On the other hand, a receiving circuit is required to amplify the current obtained from the two photodetectors. FIG. 8 shows a high impedance type receiving circuit using an FET photodetector.

In FIG. 8, 94 is two Fs for light detection.
ET, 95, 96 and 97 are other FETs, and 98 is a resistor. When manufacturing these elements, the wiring capacitance for connecting the electrodes becomes a problem, but in this embodiment, since the photodetector and the electronic element have exactly the same layer structure, an extra layer is laminated. There is no need, and the parasitic capacitance can be reduced accordingly. The step difference between the elements is 0.2
Since there is only a groove having a height of about μm, the width of the interconnect metal may be about 2 μm, and the capacity can be reduced.
Therefore, high-speed response and noise characteristics are improved. Further, since they are the same element, they can be easily manufactured. Therefore, a better SN ratio can be obtained.

This configuration is also effective when the substrate material and the material of each layer are grown on another III-V semiconductor, II-VI semiconductor, or Si substrate. Also, the structure of the waveguide is not limited to the ridge waveguide,
In the case of embedding, it is also effective in a disordered structure of a superlattice formed by diffusion ion implantation or the like.

Further, this structure is effective not only in the high impedance receiving circuit but also in other receiving circuits such as a transimpedance type receiving circuit.

[0052]

Sixth Embodiment FIG. 10 shows an embodiment in which the integrated photodetector of the present invention is used in a one-to-one coherent communication system.
This system includes a transmitter 101, a transmitter 102, and a receiver 1.
It consists of 03.

The transmitter 101 comprises a light source 111 such as a semiconductor laser and a light modulation circuit 112. The receiving unit 103 includes a polarization control circuit 131, an optical local oscillator circuit 132, and a mixing circuit 13.
3, a photodetector circuit 134, and a demodulation / identification circuit 135. The integrated photodetector according to the first or second embodiment of the present invention is used for the photodetector circuit 134 and the mixing circuit 133, or these circuits 133, 134 and the optical local oscillator circuit 132. Alternatively, the integrated photodetector of the third, fourth or fifth embodiment of the present invention is used for the photodetector circuit 134.

The signal output from the transmitter 101 is transmitted to the transmitter 102.
After that, the signal enters the receiving unit 103 and the plane of polarization is controlled. Further, since the local oscillation light is oscillating in the integrated photodetector or from the optical local oscillation circuit, the signal light and the local oscillation light are mixed and branched by the mixing circuit 133 and detected by the balanced type receiver of the photodetection circuit 134. And is output as an output signal through the demodulation / identification circuit 135.

By using the integrated photodetector of the present invention, noise can be suppressed and a good SN ratio can be obtained. Therefore, long-distance communication can be performed without relay. Further, by integrating other devices (preamplifier, polarization control element, etc.), the effect is further enhanced.

[0056]

Embodiment 7 FIG. 10 shows an embodiment in which the integrated photodetector of the present invention is used in an N to N bidirectional coherent communication system.

The signals from the transmitters 141 and 142 are combined by the multiplexer / demultiplexer 143 or 144 and the optical fiber 14 which is the transmission path.
5, the receiving unit 152 through the multiplexer / demultiplexer 144 or 143,
Reach 151. N sets of transmitters and receivers are provided at both ends. Transmission units 141 and 142
The configuration of is composed of a light source and a light modulation circuit as in the sixth embodiment. The configurations of the receiving units 151 and 152 are the same as in the sixth embodiment.
Polarization plane control circuit, optical local oscillator circuit, mixing circuit, optical detection circuit,
Consists of a demodulation / identification circuit. Optical detection circuit and mixing circuit,
Alternatively, the integrated photodetector according to the first or second embodiment of the present invention is used in these two circuits and the optical local oscillator circuit. Alternatively, the integrated photodetector of the third, fourth or fifth embodiment of the present invention is used in the photodetector circuit.

From the transmitters 141, 142 to the receiver 151,
The signal light that has reached 152 is detected by the integrated photodetector as in the sixth embodiment.

By using the integrated photodetector of the present invention, noise can be suppressed and a good SN ratio can be obtained. Therefore, long-distance communication can be performed without relay. Further, in the case of the wavelength multiplexing system, the channel can be selected according to the wavelength of the local oscillation light, and the multiplicity increases. Also,
By integrating other devices (polarization control element, preamplifier, etc.), the effect is further enhanced. Further, by integrating with other optical devices and electronic devices as shown in FIGS. 7 and 8, the effect is further enhanced.

[0060]

Since the first structure of the present invention (the first and second embodiments) is structured as described above,
The following effects are achieved. 1. By monolithically integrating a balanced receiver composed of two FET structure photodetectors and a 3 dB coupler, the coupling between the two photodetectors and the 3 dB coupler can be performed uniformly and stably, The SN ratio becomes a value closer to the ideal SN ratio. 2. By monolithically integrating the balanced receiver, 3 dB coupler, and local light source (in the case of the second embodiment), a sufficiently strong local light can be stably obtained, and SN
The ratio becomes a value closer to the ideal SN ratio. 3. Since the FET structure photodetector can respond at high speed, there is an effect that the integrated photodetector configured by this can be used for high bit rate communication. 4. Since the integration with the preamplifier is easy, a better S / N ratio can be obtained, and a faster response can be achieved.

Since the second structure of the present invention (third, fourth and fifth embodiments) is structured as described above, the following effects can be obtained. 1. By monolithically integrating two FET structure photodetectors to form a balanced receiver, the uniformity of the two photodetectors is improved, and the SN ratio is close to the ideal SN ratio. There is an effect. 2. Since the FET type photodetector can respond at high speed, there is an effect that the balanced type receiver configured thereby can be used for high bit rate communication. 3. Since the integration with the preamplifier is easy, a better S / N ratio can be obtained, and a faster response can be achieved.

[Brief description of drawings]

FIG. 1 is a perspective view of an integrated photodetector according to a first embodiment.

FIG. 2 is a schematic diagram showing a configuration of heterodyne detection using a balanced receiver.

FIG. 3 is a perspective view of an integrated photodetector of a second embodiment.

FIG. 4 is a diagram showing a layer structure of an integrated photodetector of a second embodiment.

FIG. 5 is a third example constituted by a waveguide type FET structure photodetector.
FIG. 3 is a cross-sectional view of the balanced receiver of the example of FIG.

FIG. 6 is a cross-sectional view of a balanced receiver according to a fourth embodiment configured by a surface-incidence FET structure photodetector.

FIG. 7 is a sectional view of a balanced receiver according to a fifth embodiment, which is configured by integrating a waveguide type FET structure photodetector and a preamplifier.

FIG. 8 is a diagram showing a receiving circuit (high impedance type) of a balanced receiver.

FIG. 9 is a configuration diagram of a balanced receiver circuit.

FIG. 10 is a diagram showing a sixth embodiment using the integrated photodetector of the present invention in a one-to-one one-way optical communication system.

FIG. 11 is a diagram showing a seventh embodiment using the integrated photodetector of the present invention in an N-to-N bidirectional optical communication system.

FIG. 12 is a diagram showing a conventional example.

FIG. 13 is a diagram showing another conventional example.

[Explanation of symbols]

1,25,41,61,81
Substrate 2, 42, 62, 63, 82
Buffer layer 3, 5, 26, 31, 33, 43, 45, 83, 85
Cladding layer 4, 32, 44, 84 Waveguide layer 6, 223 3 dB coupler 7, 27, 34, 46, 64, 86
Active layer 11, 51, 71, 91 Source electrode 12, 52, 72, 92 Gate electrode 13, 53, 73, 93 Drain electrode 21 Photodetector 23 Local light source 47, 65, 87 Silicon nitride film 94, 95 Photodetection FETs 95, 96, 97 FETs 101, 141, 142 Transmitters 102, 145 Transmission lines 103, 151, 152 Receivers 111 Light sources 112 Optical demodulation circuits 131 Polarization plane control circuits 132 Optical local oscillator circuits 133 Mixing circuits 134 Optical detection circuits 135 Demodulation / identification circuit 143, 144 Multiplexer / demultiplexer

Continuation of front page (51) Int.Cl. ⁵ Identification number Office reference number FI technical display location // H01L 21/338 9171-4M H01L 29/80 R

Claims (12)

[Claims] 1. A balanced type receiver formed by integrating an FET structure photodetector for detecting an input light quantity by a current flowing between a source electrode and a drain electrode, and 3d.
An integrated photodetector characterized by monolithically integrating a B coupler. 2. The integrated photodetector according to claim 1, wherein the local light source is monolithically integrated. 3. The integrated photodetector according to claim 2, wherein the local light source has a wavelength tuning function. 4. The integrated photodetector according to claim 2, wherein the local light source has an output waveguide. 5. An integrated photodetector, characterized in that a balanced receiver is constructed by integrating an FET structure photodetector that detects an input light quantity by a current flowing between a source electrode and a drain electrode. .. 6. The integrated photodetector of claim 5, wherein the FET structure is a MESFET structure. 7. The integrated photodetector according to claim 5, wherein the FET structure is a JFET structure. 8. The integrated photodetector according to claim 5, wherein the FET structure photodetector is a waveguide type photodetector. 9. The integrated photodetector according to claim 5, wherein the FET structure photodetector is a surface incidence type. 10. The integrated photodetector according to claim 5, wherein the two FET structure photodetectors and a preamplifier are monolithically integrated. 11. A one-way coherent optical communication system in which a transmitting terminal station and a receiving terminal station are connected by an optical transmission line,
A coherent optical communication system, characterized in that a receiving terminal station is provided with the integrated photodetector according to claim 1 to perform heterodyne detection. 12. A bidirectional coherent optical communication system in which a transmitting terminal station and a receiving terminal station are connected by an optical transmission line,
A coherent optical communication system, characterized in that a receiving terminal station is provided with the integrated photodetector according to claim 1 to perform heterodyne detection.